How to choose and use NPK fertiliser

How do you choose an NPK fertiliser to suit your growing medium and growth phase, and how do you use it without over-fertilising your plants? We’ll go through the fertiliser label point by point, nutrient ratios for growth and flowering, and the most common fertilising mistakes. We explain what the abbreviation NPK itself and the numbers on the packaging mean in the article NPK: what is it?.

A quick reminder: what the NPK numbers mean

In the usual N–P₂O₅–K₂O labelling system, the three numbers show the nutrient content as a percentage by weight. A 4-3-6 fertiliser therefore contains 4% nitrogen, phosphorus equivalent to 3% P₂O₅ and potassium equivalent to 6% K₂O. This is an oxide conversion, not the actual form of the nutrients. Check the symbols on the label: the content may also be expressed as the elements P and K. You can find a detailed explanation, including the conversion to pure elements, in the article NPK: what is it and what do the numbers on fertiliser mean?.

How to choose an NPK fertiliser

In addition to the NPK figures, the fertiliser packaging contains other information to help you determine whether it is suitable for your plants:

• Growing medium: fertilisers differ for soil, coco and hydroponics. Choose a range designed for the medium you grow in: fertilisers for soil, for coco or for hydroponics.
• Growth phase: a growth, flowering or universal fertiliser (see the table below).
• Organic or mineral: nutrients in organically bound form are generally made available through the activity of microorganisms; however, organic fertilisers can also contain nutrients that are readily available. Soluble mineral fertilisers act quickly, but mineral fertilisers with controlled release are also available. We describe the differences in the article organic vs mineral fertilisers.
• Detailed composition: alongside NPK, look for a breakdown of other nutrients (magnesium, calcium and trace elements) and forms of nitrogen, such as nitrate, ammonium, urea or organically bound nitrogen. The need for supplements depends on the actual composition, water, substrate and the plants’ requirements.
• Single-component or A+B: some mineral salts precipitate when concentrated, for example calcium salts with phosphates or sulphates. This is why some fertilisers come in two separate bottles. Add them to the water separately and in the order specified by the manufacturer; do not mix the concentrates directly together.
• Dosage table: look for the dose, frequency and method of application for your medium and growth phase. For liquid concentrates, the dose is often given in ml per litre of water; other products may use different units and application schedules.

If you are just starting out, choose one complete fertiliser range from a single manufacturer and do not mix brands. You can find an overview of all types in the base nutrients category, and more tips in the article how to choose a fertiliser.

Which NPK ratio to use at each growth phase

Plants’ nutrient requirements change as they grow. However, there is no universal ratio for a particular phase: it depends on the crop, substrate and nutrient reserves. As a general guide:

Leafy vegetables and herbs grown for their leaves (lettuce, basil) are usually harvested before full flowering; they do not automatically need to be switched to a flowering range. These plants can flower too, however. Tomatoes and chili peppers have changing nutritional requirements as they produce fruit, but a higher dose of phosphorus cannot be recommended regardless of the reserves in the substrate. You can find doses for specific ranges in the fertiliser dosage tables.

NPK fertilisers in our range

An overview of ranges in our selection by growing medium and phase. The NPK values are provided by the manufacturer; for BioBizz and Growth Technology, we take them from Higarden product pages and identify them in the table. For Plagron, we use rounded NPK values from its product pages; technical data sheets give a more precise composition. The label on the specific pack is always the definitive reference.

When using BioBizz in coco, follow its schedule: the manufacturer requires pH adjustment and an enzyme product. For Plagron Cocos A+B, the manufacturer gives the NPK for each component separately. For hydroponics, choose a fertiliser from the fertilisers for hydroponics category; you can find the full range in the base nutrients category.

How to use NPK fertiliser

• Follow the manufacturer’s dosage instructions. The amount on the packaging is a recommendation for the specified conditions, not automatically the maximum for all plants. For seedlings or sensitive plants, use the dose intended for their phase and take into account any pre-fertilisation of the substrate.
• Dilute liquid concentrate first. Do not pour it undiluted onto the roots. Apply ready-to-use products according to the label. We describe the differences between liquid and dry fertilisers in the article dry or liquid fertilisers.
• Monitor EC and pH. Measure the nutrient solution (coco, hydroponics) with an EC metre and a pH metre. EC indicates the total conductivity of dissolved ions, including salts from the source water. High EC may indicate excess salts, but it does not by itself determine the cause or the ratio of individual nutrients.
• Do not over-fertilise. Excess salts can cause leaf tips and edges to dry out and slow growth, although other causes can produce similar symptoms. If salinity is confirmed, stop fertilising; flushing with low-salt water may help in a container with free drainage. Without drainage, there is a risk of waterlogging. Adjust the next dose according to your measurements and the instructions.
• Check whether the substrate is pre-fertilised. Some soils contain nutrient reserves for the first few weeks, while others are only lightly pre-fertilised or contain no added fertiliser. The substrate manufacturer will specify when to start fertilising.

You can find a general step-by-step guide in the article how to fertilise correctly.

Frequently asked questions

Do I need to add a PK fertiliser to a complete fertiliser?

Not automatically: it depends on the composition of the base fertiliser, nutrient reserves and the plants’ needs. Follow the schedule for the specific range, as some ranges include a PK supplement. For example, BioNova recommends on its website combining Soil Supermix with PK 13-14 throughout the flowering period.

When should I start using NPK fertiliser?

For pre-fertilised soil, it depends on its nutrient reserves and the plants’ needs. In unfertilised coco and hydroponics, nutrients must be supplied in a suitable nutrient solution once the seedlings are established; the timing and strength depend on the specific method. Coco mixes may also be pre-fertilised, so coco cannot generally be described as a nutrient-free medium. Follow the instructions for both the fertiliser and the substrate.

Which NPK fertiliser is suitable for tomatoes?

Choose nutrition for tomatoes and your medium; when growing in soil, follow a soil analysis and do not automatically restrict nitrogen or increase phosphorus from the first flowering. Examples of ranges for soil are BioBizz Bio-Grow 4-3-6 and Bio-Bloom 2-7-4, according to Higarden product pages. BioBizz recommends using Bio-Grow during flowering too, when Bio-Bloom is added. BioNova Soil Supermix 7-3-6 is intended for growth and flowering, and the manufacturer recommends PK 13-14 for flowering.

Can I combine fertilisers from different manufacturers?

You can, but we recommend that beginners stick to one range and its schedule. When combining products, nutrient doses may overlap, and some concentrates are chemically incompatible. The brand itself is not what matters; it is the composition, total dose and mixing instructions.

Where to buy NPK fertiliser

Choose a base fertiliser according to your growing medium and growth phase: base nutrients for growth, for flowering, or browse our full range of fertilisers. Need advice? Write to us or visit our shop in Prague.